Add test for dates in mpl.
Accept any dateformatter! 'DateFormatter' is always in the string.

There might be a super class we could test the instance of, but
this is easy enough for now.
More stable mpl_date conversion, just needs a timezone. (see below).

pytz is a dependency of matplotlib, so this isn't adding bloat to
our optional dependency list.
Merge pull request #130 from plotly/andrew-fix-pandas-date-formatter

Andrew fix pandas date formatter